1. Field of the Invention
The present invention relates to a drive unit for a multilayer recording optical disk, and particularly relates to a layer jump control technique for moving an objective lens in a focusing direction to perform a focus servo operation with respect to an arbitrary recording layer.
2. Background Art
In recent years, an optical disk called a DVD is used as a large capacity recording medium. A recording layer of the DVD is constructed by two layers at its maximum per one face, and data can be recorded to both faces. In the drive unit for such a multilayer recording optical disk, when regeneration, etc. of another layer are required in an operating state of the focus servo with respect to the recording layer being regenerated or recorded, it is, required to have a function (layer jump function) for controlling the distance of the objective lens with respect to the optical disk in the focusing direction so as to perform the focus servo operation with respect to this layer as an object.
In general, in the layer jump, the movement of the objective lens in the focusing direction is controlled by observing a focus error signal generated on the basis of reflected light from the recording layer of the optical disk.
FIG. 6A is a waveform diagram of a standard focus error signal when the objective lens of the optical disk drive unit is moved from a position far from the two-layer recording optical disk until a position approaching the two-layer recording optical disk. In this figure, a focusing point of layer 0 shown by an arrow shows a position in which the objective lens is focused on the recording layer of a lower layer (layer 0). Further, a focusing point of layer 1 shows a position in which the objective lens is focused on the recording layer of an upper layer (layer 1).
As shown in this figure, when the objective lens begins to be moved from the position far from the optical disk, the standard focus error signal once has a peak in a high (Hi) direction, and the focusing point of layer 0 is set approximately when this standard focus error signal reaches a reference level. Thereafter, the standard focus error signal has a peak in a low (Lo) direction, and again passes through the reference level, and has a peak in the high (Hi) direction. The focusing point of layer 1 is set approximately when the standard focus error signal next reaches the reference level. When the objective lens is moved to a position near the optical disk, the standard focus error signal again has a peak in the low (Lo) direction.
FIG. 7 is a view for explaining processing for moving the objective lens on the basis of the focus error, signal in the case of a layer jump from layer 0 to layer 1.
In this figure, a high (H) comparing level and a low (L) comparing level are those of a reference signal compared with the focus error signal, and values (absolute value) of these comparing levels are set in advance. An Hc signal becomes a high (Hi) output when the value of focus error signal exceeds the high comparing level, and becomes a low (Lo) output in the other cases. On the other hand, an Lc signal becomes a high (Hi) output when the value of focus error signal exceeds an Lc comparing level or less, and becomes an low (Lo) output in the other cases.
A coil portion is arranged around the objective lens for converging a laser beam to the recording layer of the optical disk, and these members are supported by a spring so as to be vertically movable. When a kick voltage is applied to the coil as a focus drive signal, force in a direction for making the objective lens approach the optical disk is applied to the objective lens. In contrast to this, when a brake voltage is applied to the coil, force in a direction for making the objective lens separate from the optical disk is applied to the objective lens.
When the regeneration of layer 1 is required during the regeneration of layer 0, i.e., in an operating state of the focus servo with respect to layer 0, the drive unit applies the kick voltage after the drive unit turns off the focus servo. Thus the objective lens begins to be moved in the direction of the optical disk. The focus error signal is oscillated in the low (Lo) direction, and the Lc signal becomes the high level. Thereafter, it is returned to the high (Hi) direction, and the Lc signal becomes the low (Lo) level. Further, when the focus error signal is directed to the high (Hi) level and the Hc signal becomes the high (Hi) level, the movement of the objective lens is decelerated by applying the brake voltage. Thereafter, when the Hc signal becomes the low (Lo) level by passing through a peak, the application of the brake voltage is stopped. The focus servo is then turned on, and the focusing operation with respect to the layer 1 is performed, and the layer 1 begins to be regenerated.
However, the focus error signal shown in FIG. 6A is an ideal signal. Accordingly, there is a case in which this focus error signal actually has an unnecessary light component. FIG. 6B is the focus error signal when there is a difference in reflectivity between adjacent layers, and a relatively large unnecessary light component is caused on the interlayer side of a layer of larger reflectivity. (In the example of this figure, the reflectivity of layer 0 is larger than that of layer 1, and the large unnecessary light component of layer 0 is caused on the side of layer 1. Conversely, a problem is also caused when the reflectivity of layer 1 is larger than that of layer 0, and the large unnecessary light component of layer 1 is caused on the side of layer 0.)
A problem caused in a layer jump from layer 0 to layer 1 will next be explained with reference to FIG. 8 when the focus error signal as shown in FIG. 6B is drawn. In the example of this figure, since the unnecessary light component of layer 0 exceeds the high (H) comparing level, the Hc signal becomes the high (Hi) level in an unnecessary light portion. Therefore, the drive unit decelerates the movement of the objective lens by applying the brake voltage. When the Hc signal then becomes the low (Lo) level, the focus servo is turned on although there is still a distance until a focusing position of layer 1. Thus, when the focus error signal is again oscillated in the high (Hi) direction, there is a case in which the drive unit further accelerates the moved objective lens and does not stop the focusing operation at the focusing point of layer 1 since the focus servo is turned on. As a result, there is a fear that the layer jump is unsuccessful.